This invention relates to polymer processing systems which treat amorphous polyester to achieve crystallization, followed by solid phase polymerization followed by cooling. An example of such a system is found in Walsh U.S. Pat. No. 5,516,880.
In systems of the type contemplated by this invention, cold amorphous polymer material is fed to a crystallizer and, after substantial heat input, hot product is discharged. The particular apparatus for achieving the crystallization may comprise an indirect heat supply unit or a fluid bed. The Solidaire(copyright) or TorusDisc(copyright) equipment manufactured by Hosokawa Bepex of Minneapolis, Minn. are examples of an indirect heat supply unit which may be utilized for achieving the crystallization. In such a system, steam or other heated fluid is passed through rotors and/or jackets which are in contact with the agitated polymer material. The polymer is thereby heated to the necessary temperature for achieving the crystallization reaction.
In a fluid bed system, for example units manufactured by Howokawa Bepex, heated air is brought into contact with the polymer material in order to improve heat transfer and to achieve the temperatures required for the crystallization reaction.
In systems of the type described, the crystallized material is discharged in the post crystallizer/preheater to further heat the crystallized material to the polymerization reaction temperature and then preheated polymer is transferred to a reactor for achieving polymerization. Subsequent to the polymerization, the material is transferred to a cooler which may also comprise, for example, a TorusDisc(copyright) or fluid bed system. Where a fluid bed cooler is employed, it has been the practice to achieve the cooling, at least in part, by contacting the hot material with cool gas which may be air or some other gas such as nitrogen. As a result, the gas is heated considerably and may then be recirculated only if first cooled. The expense of gas recirculation and cooling is, however, only warranted where nitrogen or some other more expensive gas is being used.
This invention provides a system for polymer processing wherein greatly improved efficiencies are achieved in the operation of the cooling portion of the cycle. In particular, the system constitutes a process and apparatus for utilization of water spray cooling in a fluid bed cooling operation which greatly improves the efficiency of this operation.
The system of the invention comprises, in particular, an arrangement wherein solid phase polymerization is achieved by introducing cold amorphous polymer to a crystallizer to crystallize the polymer. The crystallized polymer is then heated to the required polymerization reaction temperature in the post crystallizer/preheater and then discharged to a reactor for annealing to achieve polymerization of the polymer. As a preferred form of the invention, dry and cool gas is introduced in a transition zone at the bottom of the reactor.
The polymer product of the reactor is discharged to a fluid bed cooler for cooling of the polymerized product. The fluid bed cooler includes an inlet for the hot polymer and an inlet for the cooling gas to provide for contact of the gas with the polymer. The cooler includes a bed portion in which the polymer moves while in contact with the cooling gas, and an upper chamber which collects the off-gas. The temperature of the polymer gradually decreases from the location of the inlet for the polymer to the location of the discharge for the polymer.
The fluid bed cooler of the present invention includes one or up to several water spray nozzles located just above or below the fluidized bed level of the polymer. The number of spray nozzles used depends generally on the size of the system involved with this fluid bed spray cooler system, the bulk of the cooling can be done by vaporization of water and, due to the high heat transfer rate of direct water vaporization, the size of the cooler can be reduced to a great extent. Accordingly, the flow rate of the fluidizing cooling gas required is considerably lower than in the regular gas-cooled fluid bed cooler, and substantial energy savings can be obtained.
The water spray cooling can be utilized in an open-loop-one-pass-gas fluid bed cooler as well as in the closed loop cooling operations where recirculation of fluidizing cooling gas is provided. In the closed loop cooler a gas-cooling-heat exchanger/condenser can be employed and the condensed water can be delivered back to the spray nozzles.